Multi-color monolithic light-emitting diodes and methods for making the same

ABSTRACT

A process for producing a light emitting diode device, the process including: forming a plurality of quantum dots on a surface of a layer including a first area and a second area; exposing the first area of the surface to light having a first wavelength while exposing the first area to a first etchant that causes the quantum dots in the first area to be etched at a first etch rate while the quantum dots have a dimension at or greater than a first threshold dimension; exposing the second area of the surface to light having a second wavelength while exposing the second area to a second etchant that causes the quantum dots in the second area to be etched at a third etch rate while the quantum dots have a dimension at or greater than a second threshold dimension; and processing the etched layer to form the LED device.

TECHNICAL FIELD

This disclosure generally relates to light emitting devices and methodsof making the light emitting devices.

BACKGROUND

In its simplest form, a light-emitting diode (LED) is a light sourcecomposed of a p-n junction diode that emits light when activated. When asuitable voltage is applied to electrical contacts on opposing sides ofthe junction, electrons are able to recombine with holes within anactive region of the device, releasing energy in the form of photons.The wavelengths of the emitted light and correspondingly, the color ofthe LED for visible wavelengths, is generally determined by the energyband gap of the semiconductor. LEDs are often small devices (e.g., lessthan 1 mm²) and integrated optical components may be used to shape theradiation pattern.

Generally, LEDs can be formed from inorganic or organic semiconductormaterials. An inorganic LED (ILED) is formed using inorganic materialssuch as compound semiconductors (e.g., InGaAs, InGaN). Compared toorganic LEDs (OLEDs) formed using organic materials, ILEDs are capableof generating light at a significantly higher power efficiency at abrightness per unit area that is several orders of magnitude higher(e.g., 1,000×˜10,000×) than that of OLEDs.

Due to the high efficiency of ILED devices, extremely small inorganicLEDs can yield sufficient light to be practically useful forapplications like displays or general illumination sources. LED dieshaving active areas of 100 μm² or less and a thickness of 10 μm or lesscan generate light visible to the human eye at drive currents on theorder of tens of nanoamps. Such devices are often referred to asmicroLEDs, mLEDs, or μLEDs. Generally, microLEDs have an active area ina range from 1 μm² to about 2500 μm² and are manufactured usingconventional semiconductor manufacturing techniques. As a result,microLEDs can have many different geometries, as specific applicationsdemand.

As noted above, the wavelength emitted by an LED is primarily set by thematerial composition, e.g., the compound semiconductor forming thediode. Additionally, depositing multiple layers of the compoundsemiconductor is often difficult or infeasible for variety of reasonsincluding manufacturing process complexity and cost considerations.Therefore, fabricating ILEDs emitting at different wavelengths (e.g.,red, green, blue) on a common substrate (“monolithic integration”) toform an integrated device may be a challenge.

SUMMARY

This disclosure features LED devices that monolithically integrateinorganic light emitting diodes (ILEDs) emitting light at two or moredistinct wavelength bands (e.g., different colored light), and methodsfor fabricating such devices.

One way of controlling emission wavelengths of a compound semiconductoris by forming a nanoparticle from the light-emitting material andcontrolling the dimension of the nanoparticle. Such a nanoparticle iscommonly referred to as a quantum dot. A quantum dot typically rangesfrom 2 nm to 50 nm in size, in which quantum confinement of electrons orholes affect the emission and absorption wavelength of the constituentmaterial. By depositing a layer of quantum dots of a particular size,followed by a selective modification of the size of the quantum dots(e.g., by etching or growth) at various regions of the substrate, ILEDshaving different emission wavelengths can be formed on the samesubstrate without depositing multiple layers of compound semiconductorsfor each wavelength.

In some embodiments, selective modification of the size of quantum dotsis achieved using photoelectrochemical (PEC) etching. PEC etching is alight-induced electrochemical etching of semiconductors, where regionsof a material that are exposed to and absorb the illuminated light areselectively etched. The electron-hole pair generated by the absorptionof light allows the material to be etched, so illuminated areas that donot absorb the light do not get etched. As absorption wavelengths of thequantum dots change with the size of the quantum dots, etching of thequantum dots causes a corresponding change in the absorption wavelength.As quantum dots become smaller in size, the absorption wavelengthdecreases. Therefore, for a fixed illumination wavelength, the etchingwill effectively self-terminate once the size of the quantum dots areetched down past a size corresponding to the illumination wavelength,enabling precise control over the absorption wavelength, and thereforethe emission wavelength, of the quantum dots.

Another way of controlling the size of the quantum dots is throughin-situ control of the growth rate of the quantum dots, utilizing theillumination-controlled growth process. Absorption of light by thequantum dots affects the balance between adsorption and desorptionduring the quantum dot growth process. As a result, photoabsoprtion(i.e., absorption of illuminated light) can cause a decrease in growthrate, and in certain conditions, reduce the growth rate to an extentthat practically stops, or clamps, the growth of the quantum dot. Asabsorption wavelengths of the quantum dots change with the size of thequantum dots, growth causing an increase in the size of the quantum dotscauses a corresponding change in the absorption wavelength. As quantumdots grows in size, the absorption wavelength increases. Therefore, fora fixed illumination wavelength, the growth will slow down, or stop alltogether, once the size of the quantum dots reaches a size correspondingto the illumination wavelength, enabling precise control over theabsorption wavelength, and therefore the emission wavelength, of thequantum dots.

In general, in a first aspect, the invention features a process forproducing a light emitting diode (LED) device, the process including:forming a plurality of quantum dots on a surface of a layer including afirst area and a second area; exposing the first area of the surface tolight having a first wavelength while exposing the first area to a firstetchant that, due at least in part to an interaction between the lightat the first wavelength and the quantum dots, causes the quantum dots inthe first area to be etched at a first etch rate while the quantum dotshave a dimension at or greater than a first threshold dimension, and ata second etch rate while the quantum dots have a dimension less than thefirst threshold dimension, the first etch rate being higher than thesecond etch rate; exposing the second area of the surface to lighthaving a second wavelength shorter than the first wavelength whileexposing the second area to a second etchant that, due at least in partto an interaction between the light at the second wavelength and thequantum dots, causes the quantum dots in the second area to be etched ata third etch rate while the quantum dots have a dimension at or greaterthan a second threshold dimension, and at a fourth etch rate while thequantum dots have a dimension less than the second threshold dimension,the second threshold dimension being smaller than the first thresholddimension, the third etch rate being higher than the fourth etch rate;and processing the etched layer to form the LED device, wherein thequantum dots in the first area are sized to emit light substantiallywithin a first band of wavelengths and the quantum dots in the secondarea are sized to emit light substantially within a second band ofwavelengths different from the first band of wavelengths.

Embodiments of the system can include one or more of the followingfeatures. For example, the exposing the first area of the surface tolight having a first wavelength while exposing the first area to thefirst etchant can include: applying a first voltage between the firstetchant and the plurality of quantum dots, and the exposing the secondarea of the surface to light having the second wavelength shorter thanthe first wavelength while exposing the second area to the secondetchant can include: applying a second voltage between to the secondetchant and the plurality of quantum dots.

In some embodiments, the exposing the first area of the surface to lighthaving a first wavelength while exposing the first area to the firstetchant can include: measuring a first current between the first etchantand the plurality of quantum dots; determining that the first current isequal to or less than a first threshold current; and based on thedetermination that the first current is equal to or less than the firstthreshold current, stopping the exposure of the surface to light havingthe first wavelength, and the exposing the second area of the surface tolight having the second wavelength shorter than the first wavelengthwhile exposing the second area to the second etchant can include:measuring a second current between the second etchant and the pluralityof quantum dots; determining that the second current is equal to or lessthan a second threshold current; and based on the determination that thesecond current is equal to or less than the second threshold current,stopping the exposure of the surface to light having the secondwavelength.

In some embodiments, the exposing the first area of the surface to lighthaving a first wavelength while exposing the first area to a firstetchant can include: tuning the first wavelength from an initial firstwavelength to a final first wavelength during the etching of the firstarea. The exposing the second area of the surface to light having asecond wavelength shorter than the first wavelength while exposing thesecond area to a second etchant can include: tuning the secondwavelength from an initial second wavelength to a final secondwavelength during the etching of the second area.

In some embodiments, the exposing the first area of the surface to lighthaving a first wavelength while exposing the first area to a firstetchant can include: illuminating only the first area of the surface tolight having the first wavelength through patterned illumination. Theexposing the second area of the surface to light having a secondwavelength shorter than the first wavelength while exposing the secondarea to a second etchant can include: illuminating only the second areaof the surface to light having the second wavelength through patternedillumination.

In some embodiments, the exposing the first area of the surface to lighthaving a first wavelength while exposing the first area to a firstetchant can include: depositing a first mask layer; and patterning anopening on the first mask layer, the opening corresponding to the firstarea. The exposing the second area of the surface to light having asecond wavelength shorter than the first wavelength while exposing thesecond area to a second etchant can include: depositing a second masklayer; and patterning an opening on the second mask layer, the openingcorresponding to the second area. The plurality of quantum dots and boththe opening on the first mask layer and the opening on the second masklayer can be on opposite sides of the layer.

The first etchant and the second etchant can be liquids. The firstetchant and the second etchant can include one or more of H2SO4, H2O2,H2O, HCL, C2H2O4, 4,5-dihydroxy-1,3-benzene disulfonic acid, hydrofluoric acid, tetrabutylammonium fluoroborate (TBABF4), KOH, H3PO4, orNaH2PO4. In some embodiments, the first etchant and the second etchantcan further include oxidizing agents.

The first etchant and the second etchant can be gases. The first etchantand the second etchant can include one or more of Cl2, BCl3, SF6, CF4,CH4, CHF3, O2, H2, N2, Ar, or He. The forming the plurality of quantumdots, the exposing the first area, and the exposing the second area canbe performed under vacuum without breaking the vacuum.

In some embodiments, the process can further include: prior to theexposing the first area of the surface to light having a firstwavelength while exposing the first area to the first etchant, etchingthe quantum dots in the first and second areas to have an initialdimension at or greater than the first threshold dimension.

The processing the etched layer to form the LED device can include:forming a first anode corresponding to the first area; forming a secondanode corresponding to the second area; and forming a shared cathodecorresponding to both the first and second areas.

In some embodiments, the plurality of quantum dots can be a firstplurality of quantum dots and the layer can be a first layer, and theprocess can further include: forming a second plurality of quantum dotson a surface of a second layer, the second layer and the first pluralityof quantum dots arranged on opposite sides of the first layer; andexposing the surface of the second layer to light having a thirdwavelength equal to or shorter than the second wavelength while exposingthe surface of the second layer to a third etchant that, due at least inpart to an interaction between the light at the third wavelength and thequantum dots, etches the quantum dots on the surface of the second layerat a fifth etch rate while the quantum dots have a dimension at orgreater than a third threshold dimension, and at a sixth etch rate whilethe quantum dots have a dimension less than the third thresholddimension, the fifth etch rate being higher than the sixth etch rate,wherein the second plurality of quantum dots are sized to emit lightsubstantially within a third band of wavelengths shorter than the firstand second bands of wavelengths.

The forming the plurality of quantum dots can include performing one ofmetal-organic chemical vapor deposition, molecular beam epitaxy, orliquid phase epitaxy.

The forming the plurality of quantum dots can include: forming a quantumwell layer; and forming the plurality of quantum dots by etching thequantum well layer.

The plurality of quantum dots can include a light emitting materialselected from the group consisting of Gallium Arsenide (GaAs), AluminumGallium Arsenide (AlGaAs), Gallium Arsenide Phosphide (GaAsP), AluminumGallium Indium Phosphide (AlGaInP), Gallium(III) Phosphide (GaP),Gallium Arsenide Phosphide (GaAsP), Aluminum Gallium Phosphide (AlGaP),Indium Gallium Nitride (InGaN), Gallium(III) Nitride (GaN), ZincSelenide (ZnSe), Boron Nitride (BN), Aluminum Nitride (AlN), AluminumGallium Nitride (AlGaN), and Aluminum Gallium Indium Nitride (AlGaInN).

The second and fourth etch rates can be less than 5 nm/min.

In another aspect, the invention features an apparatus that includes alight emitting device produced using the process for producing a lightemitting diode (LED) device. In some embodiments, the apparatus can be adisplay.

In a further aspect, the invention features a process for producing alight emitting diode (LED) device, the process including: forming aplurality of quantum dots on a surface of a layer including a first areaand a second area, the forming of the plurality of quantum dotsincluding: exposing the first area of the surface to light having afirst wavelength while exposing the first area to a quantum dot formingenvironment that, due at least in part to an interaction between thelight at the first wavelength and the quantum dots, causes the quantumdots in the first area to form at a first growth rate while the quantumdots have a dimension less than a first threshold dimension, and at asecond growth rate while the quantum dots have a dimension at or greaterthan the first threshold dimension, the first growth rate being higherthan the second growth rate; exposing the second area of the surface tolight having a second wavelength longer than the first wavelength whileexposing the second area to the quantum dot forming environment that,due at least in part to an interaction between the light at the secondwavelength and the quantum dots, causes the quantum dots in the secondarea to form at a third growth rate while the quantum dots have adimension less than a second threshold dimension, and at a fourth growthrate while the quantum dots have a dimension at or greater than thesecond threshold dimension, the second threshold dimension being largerthan the first threshold dimension, and the third growth rate beinghigher than the fourth growth rate; and processing the layer to form theLED device, wherein the quantum dots in the first area are sized to emitlight substantially within a first band of wavelengths and the quantumdots in the second area are sized to emit light substantially within asecond band of wavelengths different from the first band of wavelengths.

Embodiments of the process can include one or more of the followingfeatures and/or features of other aspects. For example, the exposing thefirst area of the surface to light having a first wavelength whileexposing the first area to a quantum dot forming environment caninclude: depositing a first mask layer; and patterning an opening on thefirst mask layer, the opening corresponding to the first area, and theexposing the second area of the surface to light having a secondwavelength while exposing the first area to a quantum dot formingenvironment can include: depositing a second mask layer; and patterningan opening on the second mask layer, the opening corresponding to thesecond area.

In some embodiments, the exposing the first area of the surface to lighthaving a first wavelength while exposing the first area to a quantum dotforming environment can include: illuminating only the first area of thesurface to light having the first wavelength through patternedillumination. The exposing the second area of the surface to lighthaving a second wavelength while exposing the first area to a quantumdot forming environment can include: illuminating only the second areaof the surface to light having the second wavelength through patternedillumination.

The quantum dot forming environment can be a gaseous environment. Theforming of the plurality of quantum dots can be performed under vacuumwithout breaking the vacuum.

The quantum dot forming environment can be a liquid environment.

In some embodiments, the forming a plurality of quantum dots on asurface of a layer including a first area and a second area can furtherinclude: prior to the exposing the first area of the surface to lighthaving a first wavelength while exposing the first area to a quantum dotforming environment, forming the quantum dots in the first and secondareas to have an initial dimension at or less than the first thresholddimension.

The light having the first wavelength and the light having the secondwavelength can be monochromatic light.

The light having the first wavelength and the light have the secondwavelength can be generated by a laser source.

The processing the layer to form the LED device can include: forming afirst anode corresponding to the first area; forming a second anodecorresponding to the second area; and forming a shared cathodecorresponding to both the first and second areas.

In some embodiments, the plurality of quantum dots can be a firstplurality of quantum dots and the layer can be a first layer, and theprocess can further include: forming a second plurality of quantum dotson a surface of a second layer, the second layer and the first pluralityof quantum dots arranged on opposite sides of the first layer, by:exposing the surface of the second layer to light having a thirdwavelength equal to or shorter than the first wavelength while exposingthe surface of the second layer to the quantum dot forming environmentthat, due at least in part to an interaction between the light at thethird wavelength and the quantum dots, forms the quantum dots on thesurface of the second layer at a fifth growth rate while the quantumdots have a dimension less than a third threshold dimension, and at asixth growth rate while the quantum dots have a dimension at or greaterthan the third threshold dimension, the fifth growth rate being higherthan the sixth growth rate, wherein the second plurality of quantum dotsare sized to emit light substantially within a third band of wavelengthsshorter than the first and second bands of wavelengths.

The forming of the plurality of quantum dots can include performing oneof metal-organic chemical vapor deposition, molecular beam epitaxy, orliquid phase epitaxy.

The plurality of quantum dots can include a light emitting materialselected from the group consisting of Gallium Arsenide (GaAs), AluminumGallium Arsenide (AlGaAs), Gallium Arsenide Phosphide (GaAsP), AluminumGallium Indium Phosphide (AlGaInP), Gallium(III) Phosphide (GaP),Gallium Arsenide Phosphide (GaAsP), Aluminum Gallium Phosphide (AlGaP),Indium Gallium Nitride (InGaN), Gallium(III) Nitride (GaN), ZincSelenide (ZnSe), Boron Nitride (BN), Aluminum Nitride (AlN), AluminumGallium Nitride (AlGaN), and Aluminum Gallium Indium Nitride (AlGaInN).

The second and fourth growth rates can be less than 6 monolayers/minute.

In a further aspect, the invention features a light emitting diode (LED)device that includes: a base layer; a first quantum dot region supportedby the base layer, the first quantum dot region comprising a firstplurality of quantum dots having a first dimension; and a second quantumdot region supported by the base layer, the second quantum dot regioncomprising a second plurality of quantum dots having a second dimensiondifferent from the first dimension, wherein the first quantum dot regionand the second quantum dot region are disposed on a common planeparallel to the base layer, and wherein the first quantum dot region andthe second quantum dot region are non-overlapping regions.

Embodiments of the LED device can include one or more of the followingfeatures and/or features of other aspects. For example, the LED devicecan further include: a third quantum dot region supported by the baselayer, the third quantum dot region including a third plurality ofquantum dots having a third dimension different from the first andsecond dimensions, wherein during operation, the first plurality ofquantum dots emits light substantially within a red band of wavelengths,the second plurality of quantum dots emit light substantially within agreen band of wavelengths, and the third plurality of quantum dots emitlight substantially within a third band of wavelengths.

In some embodiments, the third quantum dot region and both the first andsecond quantum dot regions can be on opposite sides of the base layer,wherein during operation, the light emitted by the third plurality ofquantum dots optically pumps the first and second pluralities of quantumdots.

In some embodiments, the LED device can further include: a first anodecorresponding to the first quantum dot region; a second anodecorresponding to the second quantum dot region; and a shared cathodecorresponding to both the first and second quantum dot regions.

Among other advantages, inorganic LEDs emitting at different wavelengthcan be monolithically integrated on a common substrate using thedisclosed techniques. Emission wavelength of quantum dots can bemodified to a desired wavelength by controlling an illuminationwavelength of light used in the photoelectrochemical etching process.Number of required electrical contacts can be reduced by sharing acommon N-contact across monolithically integrated ILEDs emitting atdifferent wavelength bands. Footprint of ILEDs can be reduced due tosharing of the N-contact.

The details of one or more implementations of the subject matter of thisdisclosure are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an example of a photoelectrochemical etching system forcontrolling etching of quantum dots;

FIGS. 1B-1D show example emission/absorption spectra of quantum dots ondifferent regions of the substrate in FIG. 1A prior to the PEC etch;

FIGS. 1E-1G show evolutions of the emission/absorption spectra of thequantum dots on different regions of the substrate during the PECetching process;

FIG. 1H shows quantum dots with modified dimensions from the PEC etchingof FIG. 1A;

FIGS. 2A-2F show steps of a first example process for fabricating amulti-color monolithic LED;

FIGS. 3A-3B show another step for fabricating a multi-color monolithicLED;

FIG. 4A shows an example of a system for controlling growth of quantumdots through illumination;

FIGS. 4B-4D show example emission/absorption spectra of quantum dots ondifferent regions of the substrate in FIG. 4A prior toillumination-controlled growth;

FIGS. 4E-4G show evolutions of the emission/absorption spectra of thequantum dots on different regions of the substrate during theillumination-controlled growth;

FIG. 4H shows quantum dots with controlled dimensions from theillumination-controlled growth of FIG. 4A;

FIGS. 5A-5E show steps of a second example process for fabricating amulti-color monolithic LED; and

FIG. 6 shows a cross-sectional view of an example of an optically-pumpedmulti-color monolithic LED device.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Referring to FIG. 1A, an example of a photoelectrochemical (PEC) etchingsystem 100 for controlling etching of quantum dots is shown. The PECetching system 100 includes an etchant tank 110, a voltage source 120, afirst illumination source 130, and a second illumination source 140. Thetank 110 is filled with liquid etchant 112, and a substrate 150containing the quantum dots 160 to be etched by the PEC etching system100 is immersed in the etchant 112. The voltage source 120 iselectrically coupled with a counter electrode 122 and the quantum dots160, and applies a voltage across the etchant 112 and the quantum dots160.

Quantum dots (QD) 160 a-c, collectively referred to as quantum dots 160,are nanoparticles that typically range from 2 nm to 50 nm in size, inwhich quantum confinement of electrons or holes affect the emission andabsorption wavelength of the constituent material. The dimensions of thequantum dots 160 and the center wavelength of the emission andabsorption spectra are generally correlated, where a reduction in thedimensions results in a decrease in the center wavelength of theemission and absorption spectra, and vice versa. In the example of FIG.1A, the quantum dots 160 a-c are distributed across a first region 152,a second region 154, and a third region 156 of the substrate 150,respectively, and initially have a uniform dimension of d₃.

As the emission wavelengths of the quantum dots 160 are correlated withthe dimensions of the quantum dots 160, independent control of thedimensions of the quantum dots on different regions of the substrate 150can enable emission of a first wavelength λ₁ from the first region 152,a second wavelength λ₂ from the second region 154, and a thirdwavelength λ₃ from the third region 156. Such independent control overetching of the quantum dots 160 located at different regions of thesubstrate 150 can be achieved using the PEC etching technique.

PEC etching is a type of chemical etching of semiconductor materialsassisted by illumination of the semiconductor being etched. A chemicaletching process proceeds by sustaining a chemical reaction between thesemiconductor and the etchant 112. Examples of various chemicalreactions between the semiconductor and the etchant 112 includesoxidation and reduction of the semiconductor.

Oxidation reactions can be initiated by holes in the valence band of thesemiconductor, and reduction reactions can be initiated by electrons inthe conduction band of the semiconductor. In PEC etching, the electronsand/or holes necessary to drive the chemical reaction processes can beprovided by illuminating the semiconductor with light having photonenergies greater than the bandgap of the semiconductor. The illuminatedlight is absorbed by the semiconductor, generating electron-hole pairsthat can drive the chemical reactions to sustain etching. At levelsbelow absorption-saturation regime of the semiconductor, the generationrate of the electron-hole pairs is generally proportional to theintensity of the illuminated light. As such, intensity of the light canbe used to control the etch rate of the PEC etching process.Furthermore, at a fixed intensity of the light, the absorption strength,or absorbance, of the semiconductor affect the amount of absorbed light,hence the amount of photo-generated electron-hole pairs.

The resulting chemical reaction forms a reaction product which may ormay not be soluble in the etchant 112. The chemical etching can besustained when the reaction product is soluble in the etchant 112, whichdissolves the reaction product and continues the chemical reaction withnewly exposed semiconductor beneath the now-dissolved layer of thereaction product. In cases where the initial reaction product is notsoluble in the etchant 112, subsequent chemical reactions may benecessary to convert the non-soluble reaction product to a solublereaction product. Such subsequent chemical reactions may be driven bythe electron-hole pairs generated by photo-absorption of the illuminatedlight by the semiconductor.

Due to the PEC etching process being driven by the availability ofphoto-generated electron-hole pairs, light illumination can be used tocontrol the PEC etching process. For example, the illumination can beturned on to begin the PEC etching process, and turned off to stop thePEC etching process. Furthermore, as the quantum dots 160 absorb lightat a specific band of wavelengths corresponding to their dimensions,wavelength of the illuminated light can be controlled to provide anadditional degree of control over the PEC etching of quantum dots 160.

Referring to FIGS. 1B-1D, example emission/absorption spectra of quantumdots 160 a-c on different regions of the substrate 150 in FIG. 1A priorto the PEC etch are shown. FIGS. 1B-1D correspond to the third region156, the second region 154, and the first region 152, respectively. TheX-axes corresponds to wavelength λ, and the Y-axes correspond torelative magnitude of emission (E) and absorption (A) by the quantumdots 160. The emission/absorption spectrum of quantum dots 160 typicallyhave a Gaussian shape with an associated center wavelength and afull-width at half maximum (FWHM) linewidth. As the quantum dots 160across all three regions all have the same dimension d₃, theirabsorption spectra are each centered around a third wavelength λ₃corresponding to the dimension d₃ of the quantum dots. In this example,the third wavelength λ₃ is the third target emission wavelength, and λ₃is longer than the remaining first and second target emissionwavelengths denoted λ₁ and λ₂.

Referring back to FIG. 1A, the first illumination source 130 generatesfirst light 132 at the first illumination wavelength λ_(1p) and thesecond illumination source 140 generates second light 142 at the secondillumination wavelength λ_(2p). The first light 132 illuminates thefirst region 152 of the substrate 150 and the second light 142illuminates the second region 154. The third region 156 is notilluminated in this example. The PEC etching process is initiated byturning on the first and second illumination sources 130 and 140 toilluminate regions 152 and 154 of the substrate 150 and turning on thevoltage source 120 and apply a bias voltage across the etchant 112 andthe quantum dots 160.

Referring to FIGS. 1E-1G, evolutions of the emission/absorption spectraof the quantum dots 160 a-c on different regions of the substrate 150during the PEC etching process are shown. The X-axes corresponds towavelength λ, and the Y-axes correspond to relative magnitude ofemission (E) and absorption (A) by the quantum dots. Referring to FIG.1E corresponding to the third region 156, the emission/absorptionspectrum of the quantum dots 162 of the third region 156 remainssubstantially unchanged at wavelength λ₃ during the course of the PECetching process, as the third region 156 does not get etched by the PECetching process due to the region not being illuminated.

Referring to FIG. 1F corresponding to the second region 154, theemission/absorption spectrum of the quantum dots 160 b of the secondregion 154 shifts during the PEC etching process, from the thirdwavelength λ₃ to the second wavelength λ₂. Initially, the quantum dots160 b each have a dimension d₃ that corresponds to the third wavelengthλ₃. The second light 142 that illuminates the second region 154 isabsorbed by the quantum dots 160 b, which causes the quantum dots 160 bto be etched.

Etching of the quantum dots 160 b leads to a decrease in the dimensionsof the quantum dots 160 b, leading to a corresponding shift in theirabsorption spectrum toward the shorter wavelengths, e.g., towards λ₂, asindicated by the arrow. As the PEC etching process continues, the centerof the absorption spectrum of the quantum dots 160 b shifts past thewavelength λ_(2p) of the second light 142, at which point the PECetching of the quantum dots 160 b begins to slow down. The PEC etchingof the quantum dots 160 b comes to a stop, or self-terminates, when theedge of the absorption spectrum of the quantum dots 160 b moves past thewavelength λ_(2p) of the second light 142, at which point the secondlight 142 is no longer absorbed by the quantum dots 160 b and thephoto-generation of the electron-hole pairs stop.

Similarly, referring to FIG. 1G corresponding to the first region 152,the emission/absorption spectrum of the quantum dots 160 a of the firstregion 152 shifts during the PEC etching process, from the thirdwavelength λ₃ to the first wavelength λ₁. Initially, the quantum dots160 a have a dimension d₃ that corresponds to λ₃. The first light 132that illuminates the first region 152 is absorbed by the quantum dots160 a, which causes the quantum dots 160 a to be etched.

Etching of the quantum dots 160 a leads to decrease in the dimensions ofthe quantum dots 160 a, leading to a corresponding shift in theirabsorption spectrum toward the shorter wavelengths, e.g., towards λ₁, asindicated by the arrow. As the PEC etching process continues, the centerof the absorption spectrum of the quantum dots 160 a shifts past thewavelength λ_(1p) of the first light 132, at which point the PEC etchingof the quantum dots 160 a begins to slow down. The PEC etching of thequantum dots 160 a comes to a stop, or self-terminates, when the edge ofthe absorption spectrum of the quantum dots 160 a moves past thewavelength λ_(1p) of the first light 132, at which point the first light132 is no longer absorbed by the quantum dots 160 a and thephoto-generation of the electron-hole pairs stop.

Referring to FIG. 1H, quantum dots 160 a-c with modified dimensions fromthe PEC etching of FIG. 1A are shown. As a result of the spatialselectivity of the PEC etching achieved by selective illumination of thedifferent regions of the substrate 150, the dimensions of the quantumdots 160 a of the first region 152 have been reduced to d₁, and thedimensions of the quantum dots 160 b of the second region 152 have beenreduced to d₂. Additionally, due to the self-terminating nature of thePEC etch-based control of the dimensions of the quantum dots, finaldimensions of the quantum dots can be accurately controlled and a tightspread in the dimensions of the quantum dots can be achieved.

Referring back to FIG. 1F, the illumination wavelength λ_(2p) of thesecond light 142 is offset by Δλ to the right of the target secondwavelength λ₂ of the quantum dots 160 b. For example, the offset Δλ canbe set such that the illumination wavelength λ_(2p) of the second light142 is set to be just past the edge of the absorption spectrum of thequantum dots 160 b. This offset Δλ between the target wavelength of thequantum dots 160 b and the illumination wavelength λ_(2p) can bedetermined from the shape of the absorption spectrum of the quantum dots160. For example, for a Gaussian-shaped absorption spectrum, the offsetcan be calculated based on the FWHM (e.g., 0.5*FWHM, 1*FWHM, 1.5*FWHM,2*FWHM, and 3*FWHM). As another example, the offset can be determined bycalculating the wavelength at which the absorption is less than acertain percentage of the peak absorption at the center wavelength(e.g., 10%, 5%, 2%, 1%, and 0.5% of the peak absorbance).

As such, while an asymptotic case where PEC etching of the quantum dots160 self-terminates has been described, in general, the PEC etchingprocess can have a first etch rate while the quantum dots have adimension at or greater than a threshold dimension, and at a second etchrate when the quantum dots have a dimension less than the thresholddimension. For example, the threshold dimension can be the dimension ofthe quantum dots 160 at which the center of the emission spectrum of thequantum dots 160 is located at the target emission wavelength. Bysetting the illumination wavelength to be at a certain offset Δλ fromthe target emission wavelength, the absorption of the illuminated lightby quantum dots 160 can be reduced to a sufficiently low level when thequantum dots 160 reach the threshold dimension. At this point, dependingon the criteria used in setting the offset Δλ, the etch rate of thequantum dots 160 is reduced to the second etch rate that is lower thanthe first etch rate when the quantum dots have a dimension at or greaterthan the threshold dimension. For example, the second etch rate can be 5nm/min or less, 4 nm/min or less, 2 nm/min or less, 1 nm/min or less,0.5 nm/min or less, such as about 0.1 nm/min.

In general, the first etch rate and the second etch rate are not fixedrates, but can be ranges of etch rates. As the etch rate of the PECetching process is affected by the amount of photo-generated carriers,which is in turn related to the amount of absorbed light, the etch ratescan change as the dimensions of the quantum dots 160 change during thePEC etching process. As such, the first etch rate can have a range ofetch rates that does not overlap with a range of etch rates for thesecond etch rate.

The control over PEC etching of the quantum dots 160 can be furtherimproved by measuring a flow of current between the quantum dots 160 andthe etchant 112. In some implementations, the voltage source 120 can bea source-meter capable of simultaneously outputting a voltage andmeasuring a current. Alternatively, an ammeter can be placed in serieswith the voltage source 120 to measure the current. As PEC etching is anelectrochemical reaction that involves oxidation and/or reduction of thesemiconductor material being etched, the current flow between thequantum dots 160 and the etchant 112 corresponds to the etch rate of thequantum dots 160. Furthermore, the total amount of charge flow betweenthe quantum dots 160 and the etchant 112, which can be determined byintegrating the current flow over the etching duration, corresponds tothe total amount of material etched. In some implementations, thevoltage source 120 can be a pulsed voltage source capable of outputtingvoltages pulses. Pulsing of voltage may be used to modifycharacteristics of the PEC etching process.

As the measured current flow indicates the etch rate of the quantum dots160, the current flow can be compared against a threshold current todetermine when to stop the PEC etching process such that the targetdimension, and hence the target emission wavelength, of the quantum dots160 is achieved. The threshold current can be set, for example, as afixed value. For example, the threshold current can be 1 μA, 2 μA, 5 μA,10 μA, 20 μA, 50 μA, and 100 μA. Alternatively, the threshold currentcan be set as a percentage relative to the peak current flow measuredduring the PEC etching process. For example, the peak current maycorrespond to the etch rate when the center of the absorption spectrumof the quantum dots 160 being etched reaches the illuminationwavelength, after which the etch rate, and hence the current flow, willdecrease due to the shifting of the absorption spectrum past theillumination wavelength. As such, the relative definition of thethreshold current may be more robust against various sources of processvariation in the PEC etching process. Examples of sources of processvariation can include temperature, etchant condition, density of quantumdots, area of the substrate, and illumination intensity. Furthermore, insome cases, the threshold current can be a threshold current densitynormalized relative to the area of counter electrode 122 or thesubstrate 150.

Additionally, or alternatively, photoluminescence from the quantum dotsunder illumination can be used to monitor the progress of the PECetching. For example, when the quantum dots 160 absorb the illuminatedlight, the quantum dots 160 may emit light in response asphotoluminescence. This magnitude of photoluminescence, for example, canbe used as an indication of whether the illumination is being absorbedby the quantum dots 160. As such, PEC etching of the quantum dots 160can be terminated, for example, when the magnitude of photoluminescencefalls below a threshold value.

The first and second illumination sources 130 and 140 can be, forexample, fixed-wavelength laser sources, tunable laser sources, pulsedlaser sources. The narrow linewidth (e.g., 0.1 nm, 1 nm) of the lightgenerates by such laser sources may be advantageous in achieving a tightdistribution in dimensions of the PEC-etched quantum dots 160. While twoillumination sources 130 and 140 have been described, in general, thePEC etching process can be performed with a single tunable illuminationsource. In such cases, PEC etching of different regions of the substrate150 can be performed sequentially by sequentially illuminating thedifferent regions.

Depending on the shape of the absorption spectrum of the quantum dots160 and the amount of wavelength shifting of the quantum dots to beperformed, it may be necessary to tune the wavelength of the first light132 and/or the second light 142 during the PEC etching process. Forexample, when the wavelength of the first light 132 is significantlyshorter than the center of the absorption spectrum of the quantum dots160 a such that falls outside of the absorption spectrum, the wavelengthof the first light 132 can be shifted to overlap with the absorptionspectrum to initiate the PEC etching process. The wavelength of thefirst light 132 can then be shifted toward the final wavelength as theetch progresses until the illumination wavelength reaches the wavelengthλ_(1p) corresponding to the target wavelength λ₁ of the quantum dots 160a. At this point, the illumination wavelength can remain fixed atλ_(1p), and the PEC etching process can proceed to completion.Alternatively, in some implementations, the first light 132 can be abroadband light having wavelengths spanning from λ1_(p) to λ3 or beyond.Such broadband illumination with a sharp cutoff at λ1_(p) can be used toenable etching of quantum dots 160 a with absorption spectra that mightotherwise fall outside of a narrowband first light 132. Such broadbandlight can be generated, for example, using a broadband light sourcefollowed by an optical high-pass filter or band-pass filter. As anotherexample, a broadband light for such purpose can be approximated bycombining multiple wavelengths, such as combining λ3, λ2, and λ1_(p) toprovide overlap between the first light 132 and the absorption spectrumof the quantum dots 160 a throughout the PEC etching process.

While emission spectrum and absorption spectrum of the quantum dots havebeen described to be equal in shape and wavelength, in general, the twospectra can be different in shape and/or center wavelength. For example,for some materials, quantum dot dimension, or combination thereof, theabsorption spectrum can be asymmetric with an extended tail toward theshorter wavelengths. In such cases, PEC etching may be performed with afixed-wavelength illumination even for wavelengths that would otherwisefall outside of a Gaussian-shaped absorption spectrum. Furthermore, insome cases, the center of the emission spectrum and the center of theabsorption spectrum may be separated by an offset wavelength. In suchcases, the offset wavelength can be taken into account (e.g., indetermining the illumination wavelength) in the PEC etching process suchthat the final dimensions of the quantum dots 160 after the PEC etchingprocess correspond to the target emission wavelength.

The etchant 112 can include various chemistries suitable for etching ofvarious semiconductors. Furthermore, the various chemistries can bemixed in various proportions to achieve desired etching characteristics.For example, for PEC etching of GaAs, a mixture of H₂SO₄, H₂O₂, and H₂Omay be used in ratio of 1:1:25-100, respectively. As another example,for PEC etching of AlGaAs, a mixture of HCL and H₂O may be used in ratioof 1:20, respectively. As yet another example, for PEC etching of InGaNand GaN, a 0.2M solution of H₂SO₄ may be used. As yet another example,for PEC etching of AlGaInN, oxalic acid (C₂H₂O₄) may be used. Otherexamples of chemistries include 4,5-dihydroxy-1,3-benzene disulfonicacid, hydro fluoric acid, tetrabutylammonium fluoroborate (TBABF₄), KOH,H₃PO₄, and NaH₂PO₄.

The counter electrode 122 can be formed from various conductive materialcompatible with the chemistry of the etchant 112. Examples of materialsfor the counter electrode 122 include platinum and gold. Additionally,in some implementations, a reference electrode can be used to provide areference potential that can be used in accurately applying the voltageacross the quantum dots 160 and the etchant 112. Examples of thereference electrode include silver/silver chloride electrode andsaturated calomel electrode.

While a voltage has been applied across the etchant 112 and the quantumdots 160 in the described example, in some implementations, PEC etchingcan be performed without applying an external voltage. Such PEC etchingprocess are referred to as “electroless process”. In an electroless PECetching process, the application of voltage across the etchant 112 andthe quantum dots 160 may be replaced with an oxidizing agent dissolvedin the etchant 112.

Referring to FIGS. 2A-2F, steps of a first example process 200 forfabricating a multi-color monolithic LED is shown. Specifically,referring to FIG. 2A, the quantum dots 160 are formed on a base layer151 supported by the substrate 150. Quantum dots 160 are typicallyformed from material capable of emitting light, such as semiconductorshaving a direct bandgap. Such semiconductor materials typically requirea high degree of crystallinity to function as an effective lightemitting material. Examples of deposition techniques suitable fordeposition of crystalline semiconductor materials and quantum dots 160include metal-organic chemical vapor deposition (MOCVD), molecular beamepitaxy (MBE), and liquid phase epitaxy (LPE).

Forming a layer of quantum dots typically requires a base layer 151 withlattice parameters suitable for the material being deposited, such thatthe quantum dot material and the base layer 151 are lattice-matched. Forexample, the substrate 150 can be a sapphire substrate, which is formedof crystalline aluminum oxide. While the sapphire substrate iscrystalline, its lattice constant is not equal to, for example, InGaNbeing deposited to form the quantum dot. As direct deposition of theInGaN material on the sapphire substrate 150 leads to crystallinedefects and consequently poor light emitting performance by thedeposited InGaN quantum dots 160, the base layer 151 can serve as abuffer layer that serves to smooth the lattice mismatch between thesubstrate 150 and the quantum dots 160. The base layer 151 can alsoinclude a charge injection layer that can facilitate injection ofelectrons or holes into the quantum dots 160 formed above to generatelight.

In some implementations, the quantum dots 160 may be formedspontaneously. For example, by controlling the deposition conditionduring the formation of the quantum dots, quantum dot 160 may be formedthrough various mechanisms such as self-assembly or nucleation as adirect product of the deposition process.

In some implementations, the quantum dots 160 may be formed by furtherprocessing a deposited layer. For example, a continuous layer of quantumwell is formed using the foregoing deposition techniques, which is thenpatterned through photolithography and etched into small islands ofquantum well, forming the quantum dots 160. The etching process, forexample, can be a PEC etching process, or conventional wet or dryetching processes. As another example, the continuous layer of quantumwell can be etched without a patterning step, and non-uniformity of thefilm and/or the etching process can be used to form the quantum dots160.

In general, different light-emitting materials have various ranges ofwavelengths over which they can emit light. As such, differentlight-emitting materials for forming the quantum dots 160 can be chosenbased on a desired emission wavelength. Examples of varioussemiconductors capable of emitting light include Gallium Arsenide(GaAs), Aluminum Gallium Arsenide (AlGaAs), Gallium Arsenide Phosphide(GaAsP), Aluminum Gallium Indium Phosphide (AlGaInP), Gallium(III)Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP), Aluminum GalliumPhosphide (AlGaP), Indium Gallium Nitride (InGaN), Gallium(III) Nitride(GaN), Zinc Selenide (ZnSe), Boron Nitride (BN), Aluminum Nitride (AlN),Aluminum Gallium Nitride (AlGaN), and Aluminum Gallium Indium Nitride(AlGaInN).

Suitable material for the substrate 150 can be chosen based on thematerial of the quantum dots 160. Examples of the material for thesubstrate include sapphire, silicon, silicon carbide, and variouscompound semiconductors such as AlN, GaN, GaAs, and InP. In cases wherethe lattice constants of the substrate and the quantum dot material aresufficiently similar, the buffer layer can be omitted. Referring to FIG.2B, a first mask layer 210 is formed over the second and third regions154 and 156. For example, a blanket film deposition followed by apatterning step, such as an lithography step and selective etching ofthe film against the quantum dots 160, can be used to form the firstmask layer 210 that has an opening that corresponds to the first region152. The first mask layer 210 serves to cover the quantum dots 160 b-cof the second and third regions 154 and 156 from light and/or etchantduring the following PEC etching step. As such, the first mask layer 210is preferably resistant to the etchant 112 used in the PEC etchingprocess in presence of illumination, and able to be removed selectiveagainst the quantum dots 160. Example of materials suitable for formingthe first mask layer 210 include photoresist, dielectrics (e.g., SiO₂,SiN, and spin-on glass), polyimide, and metals resistant to etching bythe etching chemistry (e.g., Au and Pt).

Referring to FIG. 2C, a first PEC etching step is performed. Forexample, the first PEC etching step involves immersing the substrate 150in the etchant 112 and illuminating it with the first light 132. Thequantum dots 160 a of the first region 152 are illuminated with thefirst light 132 while being exposed to the etchant 112, leading to PECetching of the quantum dots 160 a. However, the first mask layer 210blocks the etchant 112 from coming in contact with the quantum dots 160b and 160 c of the second and third regions 154 and 156. As such, thequantum dots 160 b and 160 c are not etched during this PEC etchingstep. Additionally or alternatively, for first mask layer 210 that isopaque to the first light 132, the quantum dots 160 b and 160 c arefurther shielded from the PEC etching due to their lack of exposure tothe first light 132. The first PEC etching step is completed when thequantum dots 160 a of the first region 152 have reached their targetdimension (e.g., d₁), at which point the first light 132 can be turnedoff to stop any further etching. The substrate 150 is removed from theetchant 112, and the first mask layer 210 is removed.

Referring to FIG. 2D, a second mask layer 212 is formed over the firstand third regions 152 and 156. The second mask layer 212 can be formedin a manner analogous to the description of FIG. 2B, and similarly, thesecond mask layer 212 serves to cover the quantum dots 160 a and 160 cof the first and third regions 152 and 156 from the etchant 112 and/orlight during the following PEC etching step.

Referring to FIG. 2E, a second PEC etching step is performed. The secondPEC etching step can be performed in a manner analogous to thedescription of FIG. 2C. At the conclusion of the second PEC etchingstep, the quantum dots 160 b of the second region 154 have reached theirtarget dimension (e.g., d₂).

Referring to FIG. 2F, the etched layer of quantum dots 160 having 3different dimensions are further processed to form a multi-colormonolithic LED device 250. Additional processing to form the devices 250can include, for example, an epitaxial regrowth step, and deposition ofan organic carrier transport layer (e.g., hole transport layer, electrontransport layer) to form a second carrier injection layer 220 on top ofthe quantum dots 160. Etching of the carrier injection layer 220 can beperformed to electrically separate the different regions 152, 154, and156. Cathodes 230 a-c can be formed to establish respective electricalconnections with the quantum dots 160 a-c of the respective regions. Forexample, the cathodes 230 a-c can be electrically coupled with thecarrier injection layer included in the base layer 151, which are inelectrical contact with the quantum dots 160. Anodes 240 a-c can beformed to establish respective electrical connections with the quantumdots 160 a-c of the respective regions. In some implementations, thecathodes 230 a-c form a shared cathode that is shared among thedifferent regions.

The resulting multi-color monolithic LED device 250 includes first,second, and third sub-pixels 252, 254, and 256 that can be individuallyaddressed, or powered, using the anodes 240 a-c and the cathodes 230a-c, respectively. For example, the first sub-pixel 252 can be a bluesub-pixel emitting blue light at wavelength λ₁, the second sub-pixels254 can be a green sub-pixel emitting green light at wavelength λ₂, andthe third sub-pixel 256 can be a red sub-pixel emitting red light atwavelength λ₃. By controlling the relative amount of light emitted bythe sub-pixels, the LED device 250 can emit light of different colors,which can be used, for example, as a tunable-color LED lamp, or as acolor pixel of a color LED display. In some implementations, thecathodes 230 a-c can be shared among the sub-pixels 252, 254, and 256,which can reduce the total number of independent electrical contactsfrom 6 to 4, which can be beneficial in applications where minimizingthe number of electrical contacts is beneficial. For example, in case ofa microLED, due to the extremely small footprint possible with suchdevices, the minimum achievable footprint of the microLED device may bedetermined by the minimum size of the electrical contacts. In suchapplications, reducing the number of electrical contacts can lead tofurther reduction in size of the microLED device, resulting in morecompact and potentially cheaper devices.

While fabrication of a single multi-color monolithic LED device 250using the process 200 has been described, the process 200 can be used tofabricate an LED display that includes an array of devices 250, eachcorresponding to a pixel of the LED display. For example, an array ofthe LED device 250 can be monolithically integrated on the substrate 150by fabricating a plurality of devices 250 in parallel using the process200.

While an example where the first and second mask layers 210 and 212 aredirectly covering the quantum dots has been described, in someimplementations, the first and second mask layers 210 and 212 can beformed on the backside of the substrate 150 opposite to the quantum dots160. In some cases, the material forming the first and second masklayers 210 and 212 can be difficult to remove, or the removal processcan negatively affect the quantum dots 160. For example, due toincomplete selectivity of the mask removal process, the quantum dots 160can be attacked during the mask removal process, reducing theperformance of the quantum dots 160. Additionally, or alternatively,residue of the mask layers even at the atomic scale can cause areduction in performance of the quantum dots 160. Such problems can becircumvented by forming the mask layers on the backside of thesubstrate. In cases where the substrate 150 and the base layer 151 issufficiently transparent to the illumination to allow sufficient amountof the first and second lights 132 and 142 to reach the quantum dots 160through them, illumination of the quantum dots 160 during the PECetching process through the backside of the substrate 150 can enablespatially-selective PEC etching without physically masking the quantumdots 160.

As an alternative to use of the masking layers 210 and 212, in someimplementations, patterned illumination can be used to selectivelyexpose different regions of the substrate to the illumination. Forexample, a photomask containing an image of a desired spatially-varyingillumination pattern can be used to project the desiredspatially-varying illumination pattern onto the substrate in a manneranalogous to photolithography. Additionally, various alignmenttechniques used in alignment of photolithographic layers to thesubstrate can be used to align the illumination patterns acrossdifferent PEC etching steps of the same substrate to form themulti-color monolithic LED device 250.

While anodes 240 a-c and cathodes 230 a-c have been described, in someimplementations, the polarity of these electrical terminals can bereversed. For example, the anodes 240 a-c may be cathodes 240 a-c, andthe cathodes 230 a-c may be anodes 240 a-c.

Referring to FIGS. 3A-3B, another step for fabricating a multi-colormonolithic LED is shown. Typically, quantum dots are grown in a singleforming step to have a dimension greater than the largest targetdimension of the quantum dots corresponding to the longest targetemission wavelength. Furthermore, while the dimensions of the quantumdots 160 prior to PEC etching steps have been illustrated as having auniform dimension d₃, referring to FIG. 3A, the quantum dots 360 a-c asformed by various deposition techniques typically have a distribution ofdimensions that are larger than the largest target dimension. Forexample, due to factors such as process variation and depositionnon-uniformity, quantum dots 360 a-c can have different dimensions d₄through d₆, respectively. The dimensions d₄ through d₆ are larger thanthe largest of the target dimension (e.g., d₃ of FIG. 1H).

Referring to FIG. 3B, a blanket PEC etching step of all regions of thesubstrate 150 is performed. An illumination source 330 is used togenerate light 332 to drive the PEC etching of the quantum dots 360. Thewavelength of the light 332, for example, can be λ_(3p), which is set toetch the quantum dots 360 to the largest target dimension d₃. Due to theself-terminating nature of the PEC etching step, the quantum dots 360a-c are etched down until they all reach the target dimension d₃, atwhich point the blanket PEC etching step is terminated.

Performing the blanket PEC etching step of all regions 152, 154, and 156of the substrate can reduce the durations of subsequent PEC etchingsteps for the remaining regions over individual etching of the differentregions. Additionally, the initial dimensional non-uniformity of thequantum dots 360 a-c from the deposition process can be reduced by theblanket etching step, which can improve process uniformity of thesubsequent processing steps.

The process step described in relation to FIG. 3B can be used as a partof the process for fabrication of multi-color LEDs, as well as anindependent step in fabrication process for a single-color LED. Use ofthe process step of FIG. 3B can lead to improvement in uniformity of thedimensions of the quantum dots 360, which can improve the coloruniformity of the resulting single-color LED fabricated from the quantumdots 360.

While PEC etching of the quantum dots using a liquid etchant 112 hasbeen described, PEC etching may also be performed with gas-phaseetchants in an analogous manner. Examples of chemistries for gas-phaseetchant include various mixtures of Cl₂, BCl₃, SF₆, CF₄, CH₄, CHF₃, O₂,H₂, N₂, Ar, and He.

Use of gas-phase PEC etching can enable deposition of the quantum dotsfollowed by PEC etching of the quantum dots without the substrateleaving the vacuum environment. For example, both the deposition and PECetching of the quantum dots can be performed in a single vacuum chamber.Alternatively, the deposition and etching can each be performed ondedicated deposition and etching chambers connected by a transferchamber that enable transfer of the substrate without leaving vacuum.The gas-phase PEC etching of different regions of the substrate can beperformed without masking layers by using patterned illumination.Processing of the quantum dots from deposition through PEC etchingthrough epitaxial regrowth without the quantum dots being exposed toatmosphere may improve performance of the quantum dots.

Furthermore, in some implementations, subtractive dimensional controlmay be performed in a vacuum environment or in an environment similar toa quantum dot forming environment in which the quantum dots were grown.For example, photo-enhanced thermal desorption process during, or afterthe deposition of the quantum dots may be used in in modifying thedimensions of the quantum dots.

Up to this point, post-deposition control of the dimensions of thequantum dots using the PEC etching techniques has been described. Now,techniques for controlling the dimensions of the quantum dots duringtheir growth process will be described. Referring to FIG. 4A, a system400 for controlling growth of quantum dots 460 through illumination isshown. The system 400 includes a quantum dot forming environment 410, afirst illumination source 420, a second illumination source 430, and athird illumination source 440. The substrate having different regions152, 154, and 156 is immersed in the quantum dot forming environment410.

The quantum dot forming environment 410 deposits quantum dots 460 a-c,collectively referred to as quantum dots 460, onto the substrate 150. Inthis example, quantum dots 460 are being deposited in the formingenvironment, and the quantum dots 460 have a uniform dimension of do ata particular time during the deposition process. As previouslydescribed, a base layer may be present on the substrate 150 tofacilitate formation of quantum dots 460 on the substrate 150.

The forming environment 410 is generally a gaseous or liquid environmentcontaining the various chemical precursors and/or constituent materialsused in forming the quantum dots 460. For example, when depositing thequantum dot 460 through MBE, the forming environment 410 can be amixture of physical vapor of the constituent material of the quantumdots, such as indium, gallium, and nitrogen when depositing an InGaNquantum dot. As another example, when depositing the quantum dot 460through MOCVD, the forming environment 410 can be a mixture of variouschemical precursors, such as tri-methyl-indium (TMI), tri-methyl-gallium(TMG), ammonia, and nitrogen when depositing an InGaN quantum dot. Asyet another example, when depositing the quantum dot 460 through LPE,the forming environment 410 can be molten liquid of the constituentmaterial of the quantum dots, such as liquid-phase gallium nitride fordepositing a GaN quantum dot.

The first, second, and third illumination sources 420, 430, and 440 canbe, for example, fixed-wavelength laser sources, tunable laser sources,and pulsed laser sources. The narrow linewidth (e.g., 0.1 nm, 1 nm) ofthe light generates by such laser sources may be advantageous inachieving a tight distribution in dimensions of theillumination-controlled growth of the quantum dots 460. While threeillumination sources 420, 430 and 440 have been described, in general,the illumination-controlled growth process can be performed with asingle tunable illumination source. In such cases,illumination-controlled growth of different regions of the substrate 150can be performed sequentially by sequentially illuminating the differentregions.

In general, deposition, or growth, of a layer of material on a surfaceis governed at least in part by a balance of adsorption and desorption.Adsorption is the adhesion of atoms, ions, or molecules from gas,liquid, or dissolved solid onto a surface. Desorption is the reversephenomenon of adsorption, resulting in a release of the atoms, ions ormolecules from the surface. At a given moment, a surface cansimultaneously experience both adsorption and desorption, where materialis deposited when the rate of adsorption is greater than the rate ofdesorption, and removed when the relationship is reversed.

In case of the deposition of the quantum dots 460, the presence ofphoto-generated carriers can affect the balance between adsorption anddesorption. When the quantum dots 460 absorb illuminated light andelectron-hole pairs are generated as a result, the presence of theelectrons and holes can decrease the adsorption rate, increase thedesorption rate, or combination thereof, leading to a decrease in growthrate or a complete stop in further growth when the rate of adsorption ismatched by the rate of desorption. For example, in case of MOCVD, thecombination of electron and holes at the surface can modify the rates ofvarious chemical reactions by the chemical precursors that take place atthe surface of the quantum dots. As the reaction products of thechemical precursors are adsorbed by the surface in forming the quantumdots, changing the rates of chemical reactions of the MOCVD process canlead to a change in the growth rate of the quantum dots 460. Thisphenomenon of illumination-controlled growth rate of the quantum dots460 can be used in controlling the size of the quantum dots 460 duringtheir growth.

Typically, growth of larger quantum dots leads to more strain and/ordefects, which can adversely affect the performance of the quantum dots.Once the defects have been formed, etching of the quantum dots to reducetheir dimension typically does not fully remove the created defects. Assuch, a controlled growth of the quantum dots to the desired dimensioncan lead to growth of quantum dots with lower number of defects, whichcan lead to improved performance of the quantum dots.

Referring to FIGS. 4B-4D, example emission/absorption spectra of quantumdots on different regions of the substrate in FIG. 4A prior toillumination-controlled growth are shown. FIGS. 4B-4D correspond to thethird region 156, the second region 154, and the first region 152,respectively. The X-axes corresponds to wavelength λ, and the Y-axescorrespond to relative magnitude of emission (E) and absorption (A) bythe quantum dots 460. The emission/absorption spectrum of quantum dots460 typically have a Gaussian shape with an associated center wavelengthand a full-width at half maximum (FWHM) linewidth. As the quantum dots460 across all three regions all have the same initial dimension d₀,their absorption spectra are each centered around an initial wavelengthλ₀ corresponding to the dimension d₀ of the quantum dots. In thisexample, the initial wavelength λ₀ are shorter than the target emissionwavelengths denoted λ₁, λ₂, and λ₃.

Referring back to FIG. 4A, the first illumination source 420 generatesfirst light 422 at the first illumination wavelength λ_(1p), the secondillumination source 430 generates second light 432 at the secondillumination wavelength λ_(2p), and the third illumination source 440generates third light 442 at the third illumination wavelength λ_(3p).The first, second and third light 422, 432, and 442 illuminate thefirst, second, and third regions 152, 154, and 156 of the substrate 150,respectively. The illumination of the quantum dots 460 a-c does notaffect the deposition process at this time as the quantum dots 460 a-cdo not absorb the illuminated light due to the respective absorptionspectra not overlapping with the respective illumination wavelengths.

Referring to FIGS. 4E-4G, evolutions of the emission/absorption spectraof the quantum dots 460 a-c on different regions of the substrate 150during the illumination-controlled growth are shown. The X-axescorresponds to wavelength λ, and the Y-axes correspond to relativemagnitude of emission (E) and absorption (A) by the quantum dots.Referring to FIG. 4E corresponding to the third region 156, theemission/absorption spectrum of the quantum dots 460 c of the thirdregion 156 shifts during the growth process, from the initial wavelengthλ₀ to the third wavelength λ₃ as marked by the arrow. Initially, thequantum dots 460 a each have a dimension d₀, at which the correspondingabsorption spectrum do not overlap with the illumination wavelength λ₃pthat corresponds to the third wavelength λ₃. As such, the third light442 illuminating the third region 156 is not absorbed by the quantumdots 460 c, and the quantum dots 460 c continue to grow in dimension.Eventually, the growth of the quantum dots 460 c causes their absorptionspectrum to overlap with the illumination wavelength λ_(3p) of the thirdlight 442, at which point the third light 442 begins to be absorbed inincreasing amounts, generating an increasing number of electron-holepairs. In this example, the balance between the rate of adsorption andthe rate of desorption reaches an equilibrium when the center of theabsorption spectrum coincides with the illumination wavelength λ_(3p).As a result, the quantum dots 460 c stops growing at this point and doesnot grow any further, as indicated by the crossed out arrow.

Similarly, referring to FIGS. 4F and 4G, the quantum dots 460 b growsuntil the center of their absorption spectrum reaches the illuminationwavelength λ_(2p) of the second light 432, and the quantum dots 460 agrows until the center of their absorption spectrum reaches theillumination wavelength λ_(1p) of the first light 422, at which pointthe quantum dots 460 a-c have reached their respective targetdimensions, and the growth process is completed.

Referring to FIG. 4H, quantum dots 460 a-c with controlled dimensionsfrom the illumination-controlled growth of FIG. 4A is shown. As a resultof the spatial selectivity of the illumination-controlled growth processachieved by selective illumination of the different regions of thesubstrate 150, the dimensions of the quantum dots 460 a of the firstregion 152 have grown to d₁, the dimensions of the quantum dots 160 b ofthe second region 152 have grown to d₂, and the dimensions of thequantum dots 160 c of the third region 156 have grown to d₃. Due to theself-terminating nature of the illumination-controlled growth, finaldimensions of the quantum dots can be accurately controlled and a tightspread in the dimensions of the quantum dots can be achieved.

In some implementations, monitoring photoluminescence from the quantumdots under illumination can be used to monitor the progress of thegrowth of the quantum dots. For example, when the quantum dots 460absorb the illuminated light, the quantum dots 160 may emit light inresponse as photoluminescence. This magnitude of photoluminescence, forexample, can be used as an indication of whether the illumination isbeing absorbed by the quantum dots 160. As such, growth of the quantumdots 160 can be terminated for example, when the magnitude ofphotoluminescence reaches a threshold value.

While an asymptotic case where illumination-controlled growth of thequantum dots 460 self-terminates has been described, in general, theillumination-controlled growth process can have a first growth ratewhile the quantum dots have a dimension less than a threshold dimension,and at a second growth rate when the quantum dots have a dimension at orgreater than the threshold dimension. For example, the thresholddimension can be the dimension of the quantum dots 460 at which thecenter of the emission spectrum of the quantum dots 460 is located atthe target emission wavelength. By setting the illumination wavelengthto be at a certain offset Δλ from the target emission wavelength, theabsorption of the illuminated light by quantum dots 460 can be increasedto a sufficient level when the quantum dots 460 reach the thresholddimension. At this point, depending on the criteria used in setting theoffset Δλ, the growth rate of the quantum dots 460 is reduced to thesecond growth rate that is lower than the first growth rate when thequantum dots have a dimension less than the threshold dimension. Forexample, the second growth rate can be 5 nm/min or less, 4 nm/min orless, 2 nm/min or less, 1 nm/min or less, 0.5 nm/min or less, such asabout 0.1 nm/min. As another example, the second growth rate can be 6monolayers/min or less, 5 monolayers/min or less, 4 monolayers/min orless, 3 monolayers/min or less, 2 monolayers/min or less, such as about1 monolayers/min.

In general, the first growth rate and the second growth rate are notfixed rates, but can be ranges of growth rates. As the growth rate ofthe illumination-controlled growth process is affected by the amount ofphoto-generated carriers, which are in turn related to the amount ofabsorbed light, the growth rates can change as the dimensions of thequantum dots 460 change during the illumination-controlled growthprocess. As such, the first growth rate can have a range of growth ratesthat does not overlap with a range of growth rates for the second growthrate.

Referring to FIGS. 5A-5E, steps of a second example process 500 forfabricating a multi-color monolithic LED is shown. Specifically,referring to FIG. 5A, a first mask layer 510 is formed over the firstand second regions 152 and 154. The first mask layer 510 can be formedas described in relation to FIG. 2B. In this example, the quantum dots460 a-c have been initially grown to have dimension d₁ corresponding tothe first target emission wavelength λ₁ prior to selective growth ofdifferent regions. The initial growth can be performed using theillumination-controlled growth technique. As growth rates of quantumdots 460 are typically low, a common growth step across the differentregions can lead to a reduction in total growth time and improvedprocess throughput. However, in general, the quantum dots of differentregions can be grown from scratch without performing a shared growthstep.

Referring to FIG. 5B, a first illumination-controlled growth step isperformed. For example, the first illumination-controlled growth stepinvolves immersing the substrate 150 in the quantum dot formingenvironment 410 and illuminating it with the third light 442. Thequantum dots 460 c of the third region 156 are illuminated with thethird light 442 while being exposed to the forming environment 410,leading to illumination-controlled growth of the quantum dots 460 c.However, the first mask layer 510 blocks the forming environment 410from coming in contact with the quantum dots 460 a and 460 b of thefirst and second regions 152 and 154. As such, the quantum dots 460 aand 460 b do not grow during this illumination-controlled growth step.The first illumination-controlled growth step is completed when thequantum dots 460 c of the third region 156 have reached their targetdimension (e.g., d₃), at which point the forming environment 410 isremoved to stop further growth and the first mask layer 510 is removed.

Referring to FIG. 5C, a second mask layer 512 is formed over the firstand third regions 152 and 156. The second mask layer 512 can be formedin a manner analogous to the description of FIG. 2B, and similarly, thesecond mask layer 512 serves to cover the quantum dots 460 a and 460 cof the first and third regions 152 and 156 from the forming environment410 during the following illumination-controlled growth step.

Referring to FIG. 5D, a second illumination-controlled growth step isperformed. The second illumination-controlled growth step can beperformed in a manner analogous to the description of FIG. 5B. At theconclusion of the second illumination-controlled growth step, thequantum dots 460 b of the second region 154 have reached their targetdimension (e.g., d₂).

Referring to FIG. 5E, the grown layer of quantum dots 460 having 3different dimensions are further processed to form a multi-colormonolithic LED device 550 analogous to the multi-color monolithic LEDdevice 250 of FIG. 2F. The further processing step can be performed in amanner analogous to the description of FIG. 2F.

While an example where the first and second mask layers 510 and 512 aredirectly covering the quantum dots has been described, in someimplementations, patterned illumination can be used to selectivelyexpose different regions of the substrate to the illumination aspreviously described. Use of patterned illumination can enablesimultaneous growth of quantum dots across different regions, which canlead to a reduction in total growth time and improved processthroughput.

While one example where the illumination wavelengths λ_(1p) throughλ_(3p) where the illumination wavelengths coincide with the respectivetarget emission wavelength has been described, in general, theillumination wavelengths can be offset from the target emissionwavelengths to achieve desired control over the target emissionwavelengths. Suitable offsets can be determined, among other factors,from the shape of the absorption spectrum, and the effect of thephoto-generated carriers on the adsorption and desorption rate ofparticular materials in a particular forming environment.

In some implementations, a voltage source such at the voltage source 120can be used to provide a bias voltage to the quantum dots 460 duringgrowth. Such bias voltage may be used, for example, to modifycharacteristics of the growth process or to monitor the growth process.

Referring to FIG. 6, a cross-sectional view of an example of anoptically-pumped multi-color monolithic LED device 600 is shown. Theoptically pumped multi-color monolithic LED device 600 includes pumpdiodes 610 a-c, collectively referred to as pump diodes 610, anddown-conversion elements 620 a-c, collectively referred to asdown-conversion elements 620.

The pump diodes 610 are LED devices that emit pump light at the pumpwavelength λ_(p) toward the down-conversion elements 620. Thepump-diodes 610 can be fabricated, for example, using the process 200 or500 for fabricating a multi-color monolithic LED. The dimensions of thequantum dots of the pump diodes are controlled during the fabricationprocess such that the pump-diodes emit light at wavelengths suitable foroptical pumping of the down-conversion elements 620. In general, thepump wavelength λ_(p) is shorter than the emission wavelengths of thedown-conversion elements 620. For example, the pump diodes 610 can emitlight in the ultraviolet wavelengths.

Down-conversion elements 620 are elements that absorb light at a shorterwavelength, which then re-emit light at a longer wavelength. Suchprocess of converting light from a shorter wavelength to a longerwavelength is referred to as “down-conversion”. Down-conversion elements620 can be implemented using quantum dots. For example, the describedtechniques for forming and controlling the dimension of the quantum dotscan be used in forming quantum dot-based down-conversion elements 620.For example, the element 620 a can be quantum dots having dimension d₁,which emits light at λ1. Similarly, the elements 620 b and 620 c can bequantum dots having dimension d₂ and d₃, which emits light at λ₂ and λ₃,respectively.

By turning on the pump diodes 610 a-c, respective down-conversionelements 620 a-c are optically pumped, which results in emission oflight at wavelengths λ₁ through λ₃. As such, the device 600 can emitlight of different colors, and be operated and used in manners analogousto the multi-color monolithic LED device 250 and 450.

The optically-pumped multi-color monolithic LED device 600 can befabricated, for example, by performing a process similar to the process200 or 500 for fabricating a multi-color monolithic LED, where maskingsteps and a second PEC etching step or a second illumination-controlledgrowth step are omitted. Once the pump diodes are formed, a second baselayer can be formed on top of the pump diodes, and the down-conversionelements 620 can be fabricated on the second base layer using thedescribed techniques for forming and controlling the dimension of thequantum dots. The down-conversion elements 620 may be passivated tomitigate performance degradation of the elements 620 due to, forexample, surface oxidation or contamination. Examples of the passivationprocess include deposition of a protective layer such as polyimide ordielectric layers (e.g., Al₂O₃, SiO₂, and SiN).

In some implementations, the pump wavelength λ_(p) can be the firstemission wavelength λ₁. In such cases, the first down-conversion element620 a can be omitted. For example, the pump wavelength can be a bluewavelength, which can provide the blue color of an RGB LED device. Asblue wavelength is shorter than the red and green, the blue pumpwavelength can optically pump the down conversion elements 620 b and 620c.

While processing of the substrate 150 having 3 non-overlapping regions152, 154, and 156 have been described, in general, substrates with anynumber of regions can be processed using the described techniques. Forexample, a 2-color monolithic LED device can be formed with a 2-regionprocessing, and a 4-color monolithic LED device can be formed with a4-region processing.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the invention. Accordingly, other embodimentsare within the scope of the claims.

What is claimed is:
 1. A process for producing a light emitting diode(LED) device, the process comprising: forming a plurality of quantumdots on a surface of a layer comprising a first area and a second area;exposing the first area of the surface to light having a firstwavelength while exposing the first area to a first etchant that, due atleast in part to an interaction between the light at the firstwavelength and the plurality of quantum dots in the first area, causesthe plurality of quantum dots in the first area to be etched at a firstetch rate while the plurality of quantum dots in the first area has adimension at or greater than a first threshold dimension, and at asecond etch rate while the plurality of quantum dots in the first areahas a dimension less than the first threshold dimension, the first etchrate being higher than the second etch rate; exposing the second area ofthe surface to light having a second wavelength shorter than the firstwavelength while exposing the second area to a second etchant that, dueat least in part to an interaction between the light at the secondwavelength and the plurality of quantum dots in the second area, causesthe plurality of quantum dots in the second area to be etched at a thirdetch rate while the plurality of quantum dots in the second area has adimension at or greater than a second threshold dimension, and at afourth etch rate while the plurality of quantum dots in the second areahas a dimension less than the second threshold dimension, the secondthreshold dimension being smaller than the first threshold dimension,the third etch rate being higher than the fourth etch rate; and afterexposing the first and second areas, processing the layer to form theLED device, wherein the plurality of quantum dots in the first area aresized to emit light substantially within a first band of wavelengths andthe plurality of quantum dots in the second area are sized to emit lightsubstantially within a second band of wavelengths different from thefirst band of wavelengths.
 2. The process of claim 1, wherein theexposing the first area of the surface to light having a firstwavelength while exposing the first area to the first etchant comprises:applying a first voltage between the first etchant and the plurality ofquantum dots, and wherein the exposing the second area of the surface tolight having the second wavelength shorter than the first wavelengthwhile exposing the second area to the second etchant comprises: applyinga second voltage between to the second etchant and the plurality ofquantum dots.
 3. The process of claim 2, wherein the exposing the firstarea of the surface to light having a first wavelength while exposingthe first area to the first etchant comprises: measuring a first currentbetween the first etchant and the plurality of quantum dots; determiningthat the first current is equal to or less than a first thresholdcurrent; and based on the determination that the first current is equalto or less than the first threshold current, stopping the exposure ofthe surface to light having the first wavelength, and wherein theexposing the second area of the surface to light having the secondwavelength shorter than the first wavelength while exposing the secondarea to the second etchant comprises: measuring a second current betweenthe second etchant and the plurality of quantum dots; determining thatthe second current is equal to or less than a second threshold current;and based on the determination that the second current is equal to orless than the second threshold current, stopping the exposure of thesurface to light having the second wavelength.
 4. The process of claim1, wherein the exposing the first area of the surface to light having afirst wavelength while exposing the first area to a first etchantcomprises: tuning the first wavelength from an initial first wavelengthto a final first wavelength during the etching of the first area, andwherein the exposing the second area of the surface to light having asecond wavelength shorter than the first wavelength while exposing thesecond area to a second etchant comprises: tuning the second wavelengthfrom an initial second wavelength to a final second wavelength duringthe etching of the second area.
 5. The process of claim 1, wherein theexposing the first area of the surface to light having a firstwavelength while exposing the first area to a first etchant comprises:illuminating only the first area of the surface to light having thefirst wavelength through patterned illumination, and wherein theexposing the second area of the surface to light having a secondwavelength shorter than the first wavelength while exposing the secondarea to a second etchant comprises: illuminating only the second area ofthe surface to light having the second wavelength through patternedillumination.
 6. The process of claim 1, wherein the exposing the firstarea of the surface to light having a first wavelength while exposingthe first area to a first etchant comprises: depositing a first masklayer; and patterning an opening on the first mask layer, the openingcorresponding to the first area, and wherein the exposing the secondarea of the surface to light having a second wavelength shorter than thefirst wavelength while exposing the second area to a second etchantcomprises: depositing a second mask layer; and patterning an opening onthe second mask layer, the opening corresponding to the second area. 7.The process of claim 6, wherein the plurality of quantum dots and boththe opening on the first mask layer and the opening on the second masklayer are on opposite sides of the layer.
 8. The process of claim 1,wherein the first etchant and the second etchant are liquids.
 9. Theprocess of claim 8, wherein the first etchant and the second etchantcomprise one or more of H₂SO₄, H₂O₂, H₂O, HCL, C₂H₂O₄,4,5-dihydroxy-1,3-benzene disulfonic acid, hydro fluoric acid,tetrabutylammonium fluoroborate (TBABF₄), KOH, H₃PO₄, or NaH₂PO₄. 10.The process of claim 8, wherein the first etchant and the second etchantfurther comprises oxidizing agents.
 11. The process of claim 1, whereinthe first etchant and the second etchant are gases.
 12. The process ofclaim 11, wherein the first etchant and the second etchant comprise oneor more of Cl₂, BCl₃, SF₆, CF₄, CH₄, CHF₃, O₂, H₂, Na, Ar, or He. 13.The process of claim 11, wherein the forming the plurality of quantumdots, the exposing the first area, and the exposing the second area areperformed under vacuum without breaking the vacuum.
 14. The process ofclaim 1, further comprising: prior to the exposing the first area of thesurface to light having a first wavelength while exposing the first areato the first etchant, etching the plurality of quantum dots in the firstand second areas to have an initial dimension at or greater than thefirst threshold dimension.
 15. The process of claim 1, wherein theprocessing the layer to form the LED device comprises: forming a firstanode corresponding to the first area; forming a second anodecorresponding to the second area; and forming a shared cathodecorresponding to both the first and second areas.
 16. The process ofclaim 1, wherein the plurality of quantum dots is a first plurality ofquantum dots and the layer is a first layer, and wherein the processfurther comprises: forming a second plurality of quantum dots on asurface of a second layer, the second layer and the first plurality ofquantum dots arranged on opposite sides of the first layer; and exposingthe surface of the second layer to light having a third wavelength equalto or shorter than the second wavelength while exposing the surface ofthe second layer to a third etchant that, due at least in part to aninteraction between the light at the third wavelength and the secondplurality of quantum dots, etches the second plurality of quantum dotson the surface of the second layer at a fifth etch rate while the secondplurality of quantum dots have a dimension at or greater than a thirdthreshold dimension, and at a sixth etch rate while the second pluralityof quantum dots have a dimension less than the third thresholddimension, the fifth etch rate being higher than the sixth etch rate,wherein the second plurality of quantum dots are sized to emit lightsubstantially within a third band of wavelengths shorter than the firstand second bands of wavelengths.
 17. The process of claim 1, wherein theforming the plurality of quantum dots comprises performing one ofmetal-organic chemical vapor deposition, molecular beam epitaxy, orliquid phase epitaxy.
 18. The process of claim 1, wherein the formingthe plurality of quantum dots comprises: forming a quantum well layer;and forming the plurality of quantum dots by etching the quantum welllayer.
 19. The process of claim 1, wherein the plurality of quantum dotscomprises a light emitting material selected from the group consistingof Gallium Arsenide (GaAs), Aluminum Gallium Arsenide (AlGaAs), GalliumArsenide Phosphide (GaAsP), Aluminum Gallium Indium Phosphide (AlGaInP),Gallium(III) Phosphide (GaP), Gallium Arsenide Phosphide (GaAsP),Aluminum Gallium Phosphide (AlGaP), Indium Gallium Nitride (InGaN),Gallium(III) Nitride (GaN), Zinc Selenide (ZnSe), Boron Nitride (BN),Aluminum Nitride (AlN), Aluminum Gallium Nitride (AlGaN), and AluminumGallium Indium Nitride (AlGaInN).
 20. The process of claim 1, whereinthe second and fourth etch rates are less than 5 nm/min.
 21. Anapparatus, comprising a light emitting device produced using the processof claim
 1. 22. The apparatus of claim 21, wherein the apparatus is adisplay.